The development in the artillery field, both land and sea artillery, has rendered possible projectiles with an increased range of fire, for example by means of a so-called base bleed unit. The increased range of fire is naturally desirable but it leads to increased absolute dispersion of the projectiles. This increased dispersion is very unfavourable, all the more so as a change in the threat picture has become noticeable towards a greater frequency of smaller and harder elementary targets where each elementary target has to be combatted. In order to reduce the dispersion of the projectiles, terminal correction or terminal guidance of the projectiles has been proposed. This means that a projectile is fired in a ballistic trajectory in conventional manner but at the end of the trajectory a target-seeking member and guidance electronics are activated which can lead the projectile to a hit or near hit on the target. Compared with a radical exchange of tube artillery for missiles, a system with terminally corrected projectiles is less complicated than a robot because continuous guidance is not used. Moreover, the projectile is more difficult to intercept when it follows a ballistic trajectory for great or greater portion of the flight.
Different solutions to this problem have been introduced. Conventional artillery ammunition is spin-stabilized over the whole trajectory, that is to say it has a high speed of rotation (of the order of magnitude of 300-2000 rad/sec). Solutions to the problems of terminal guidance of projectiles which are spin-stabilized over the whole trajectory have been put forward. The advantages of such a system are that a completely conventional firing can be effected with ammunition effects which differ little in size and weight from conventional ammunition. The disadvantages are the very complicated guiding and the limited range of control as well as the very uncertain possibilities of realisation.
The target seeker is complicated and considerable difficulties arise in correcting the course since the roll position of the projectile must be determined when the guiding signal is given. It has been proposed that the roll direction should be determined in relation to a reference direction by means of the so-called rate-gyro and integration. This proposal is not without problems, however, because the gyro is sensitive to acceleration and can drift. With projectiles which are fired with a gun barrel, the sensitivity to acceleration is a particularly serious problem.
Thus a projectile which is spin-stabilized is altogether unsuitable for use as a terminally guided projectile or in general if the projectile is to receive for example an explosive charge with a hollow-charge effect where the explosive radiation is adversely affected if the explosive charge rotates.
An attempt to eliminate the disadvantages of a projectile which is spin-stabilized by discharging a useful load from the projectile is disclosed in the Swedish Patent Specification No. 363 892. There a projectile which is spin-stabilized is disclosed which is provided with brake flaps which, at the desired moment in the trajectory, are lowered and brake the rotation of the projectile so that the projectile becomes unstable, after which the useful load of the projectile is thrown away. Since such a projectile thus becomes unstable as a result of the braking of the rotation, it cannot serve as a terminally guided projectile or be provided with an explosive charge with a hollow-charge effect since that would require that the projectile should be aerodynamically stable.
The majority of solutions hitherto put forward for the problem of terminal guidance mean that the projectile is provided with so-called rotating driving bands which means that the projectile has a low speed of rotation (of the order of magnitude 0-200 rad/sec) when it leaves the muzzle. This means that stabilizing fins must be extended immediately outside the muzzle. The advantages of this system with low or no speed of rotation in the trajectory is that target seeking and guiding can be fairly simple. Certain warheads, such as explosive charges with a hollow-charge effect, require a low speed of rotation to give a good result, as mentioned above. The disadvantages of this system are that the range of firing is adversely affected. Moreover, the dispersion easily increases since the projectile is sensitive to disturbances at the beginning of the trajectory, that is to say when the fins are extended, and the extension of the fins easily introduces disturbances. Moreover, with the solutions hitherto proposed, the length of the projectile has greatly exceeded that which applies to conventional projectiles, which imposes new demands on the handling of ammunition particularly where automatic loading systems are concerned.